Magnetoreception: nature's invisible map
Explore an extraordinary way animals are able to navigate
By Bruce Saleeb-Mousa
The ease with which we navigate towns, cities, continents and oceans is owing to our ability to manipulate the laws of physics and engineer tools and gadgets that help us reach our destinations. Of course, it wasn’t always this way.
Those before us faced the frequent, painstaking task of plotting a course and constantly getting lost. This is because, as humans, we rely heavily on visual aids and memory to help us navigate, whether it be landmarks or the night sky. The major problem with this, being that, over large distances, landmarks may become indistinguishable, our memory may not serve us well, or simply our navigating techniques may be inadequate. Fortunately for many critters across the animal kingdom, this is not a problem—they can use the Earth’s magnetic field to navigate land and sea.
This field is generated by the Earth’s liquid outer core. It has characteristics akin to that of a huge bar magnet placed at the Earth’s centre, oriented at around 11 degrees to the rotational axis. The strength of the field ranges from around 25 microtesla at the equator, to around 60 microtesla at the poles. In comparison, a standard fridge magnet is around a hundred times larger at about 5 millitesla—so Earth’s field is pretty weak. Thus, in order to navigate using the variation in this field, the method of sensing must be able to resolve small changes of up to ~35 microtesla.
Navigation using the geomagnetic field has been established experimentally for certain animals1. Behavioural patterns in migratory birds, for example, suggest that they use magnetic sense to find their way south in autumn, and north in spring.
The underlying mechanism involved in magnetoreception, however, is not fully understood. Three main processes may play a part: mechanical reception, where a magnetic field exerts a torque on a ferromagnetic material (the reason why a compass needle rotates); electromagnetic induction, where a change in magnetic field through a conductive material induces a voltage; or chemical reception, where variations in magnetic fields cause changes in the spin states of certain molecules. Of these three, most evidence seems to support a chemical reception mechanism2.
There is debate as to whether humans are able to sense magnetic fields3. Theories based on the mechanical reception mechanism suggest that tiny compass-like needles made of a material called magnetite sit in animal receptor cells, and that these can trigger nerve endings. The same material can be found in humans—unfortunately our ability to manipulate or even sense this seems to have been lost.
From Issue 14